1. Field of the Invention
This invention relates to radio frequency (RF) electromagnetic (EM) field resonant cavities, and more particularly to such cavities in which high RF power output is to be achieved.
2. Description of the Related Art
It is known to generate high frequency EM fields in a cavity for the purpose, e.g., of accelerating a charged particle beam in a radio frequency (RF) accelerator, such as a linear accelerator. In a typical construction of a linear accelerator, RF power is provided by a number of vacuum tube amplifiers operating at a high voltage (tens of kilovolts or higher) and the amplified RF power is transmitted from the RF tubes to the cavity by means of a coaxial cable, or the like, to form an oscillating EM field inside the cavity. One common type of linear accelerator (linac), the drift tube linac (DTL), has a series of drift tubes which are arranged within the cavity so that the particles are accelerated by the electric field to form the desired particle beam.
There are a number of applications or potential applications for which relatively light-weight and easily transportable RF cavities or linacs would be desirable. These applications include earth-orbit based applications; decentralized, on-demand production of medical isotopes; and high-power RF amplifiers, among others. However, the substantial weight and size of the necessary RF tubes, high voltage power supply and power conditioning equipment, and associated components, has been a significant deterrent to use of RF cavities or linacs for these purposes.
One proposed solution to overcome the disadvantages of the above-mentioned RF cavities is shown and described in U.S. Pat. No. 5,497,050 issued to Bernard R. Cheo on Mar. 5, 1996. FIG. 4 of U.S. Pat. No. 5,497,050, reproduced in this application as FIG. 1, shows an RF cavity 110 defined by a wall 112, which has a conductive inner surface 114. The wall 112 is divided into upper and lower cylindrical sections 120 and 122 and installed between the sections is an annular array 124 of solid state power amplifier modules 126. Each module 126 has an input terminal which is connected to a source of a relatively low power RF driving signal. A positive d.c. terminal 140 is connected to the upper section 120 at an outer surface, and similarly, a negative d.c. terminal 146 is connected to the lower section 122 via a quarter-wavelength choke connection. When used as an amplifier, the RF cavity 110 includes a waveguide 160, or alternatively, a coaxial cable output connector, for taking out high power EM waves from the cavity 110.
In operation, the RF driving input power applied to the terminals of the amplifier modules 126 is at a frequency that corresponds to that of the desired resonant mode of cavity 110. Under control of the input drive amplifier modules 126 induce a large RF current, with a peak amplitude on the order of several kiloamps, to flow at inner surface 114 of wall 112, so that the desired EM field amplitude is established. Due to skin effect, this current flows along the inside surface of the cavity wall to a depth on the order of few microns. The d.c. power supply output current which passes through the modules flows through the bulk of wall 112. The amplifier modules 126, which are low impedance devices, operate at high-current/low-voltage, while a particle beam generated along an axis of cavity 110 is at high-voltage/low-current, representing a high impedance load. Thus, the RF cavity 110 disclosed serves at once as a power combiner and a matching transformer for the amplifier modules 126.
Due to its construction, a minimal amount of packaging is required within the modules 126, because the wall 112 of cavity 110 serves as a heat sink for the transistors. The system""s total cooling budget is not increased, while most of the packaging, which makes up the heaviest part of a transistor RF power system, is eliminated. Additionally, since the vacuum tubes required for conventional RF linear accelerators are not required to be provided with accelerator cavity 110, the break-down problems of high voltage equipment in earth orbit are eliminated. Further, the power supply of the cavity 110 avoids the RF power transmission loss of conventional accelerators, thereby achieving higher efficiency.
While the cavity disclosed in U.S. Pat. No. 5,497,050 is highly effective for use as a linear accelerator, it lacks the structure for efficient operation as an amplifier. First, the cavity disclosed only shows a waveguide as the cavity""s output port with no mention of an input port for receiving the RF power which is to be amplified. Second, for any radio frequency device involving a resonant cavity, it is necessary that frequency tuning can be performed in order that it can operate properly and at the desired frequency. The prior art RF cavity disclosed by U.S. Pat. No. 5,497,050 neither addresses nor shows any means by which tuning can be achieved when acting as an amplifier.
Accordingly, it is an object of the present invention to provide a high-power RF cavity that can easily be transformed into an efficient high-power amplifier.
It is another object of the present invention to provide a high-power amplifier with discrete input and output ports.
It is a further object of the invention to provide a high-power amplifier with frequency tuning capabilities.
According to the present invention, the foregoing objects are met by the provision of an active radio frequency cavity amplifier (ARFCA). The ARFCA includes a housing defining two independently tunable resonant cavities. Each cavity is generally cylindrical and includes conductive walls. Conductive structures in a first cavity, i.e., an input cavity, couple an RF field within the input cavity to input leads of a plurality of power transistors formed in an annular array. Similarly, conductive structures in a second cavity, i.e., an output cavity, couple an RF field within the output cavity to output leads of the plurality of power transistors.
A plunger assembly is provided for the input cavity for coupling low RF power from a source into the input cavity. A plunger assembly is also provided for the output cavity for coupling the amplified RF power out to a load. The plunger assembly of each cavity further serves as a mechanism for tuning the cavities to resonate at the desired operating frequency.
The ARFCA in accordance with the present invention is a relatively low weight device, using a low voltage DC power source for the RF power transistors. The input cavity functions as a power distributor and matching transformer to the input of a large number of RF power transistors. The output cavity serves as the power combiner and the matching transformer from the output of the same transistors. The walls of the cavities can serve as a heat sink. High combining efficiency is achieved.
According to an aspect of the invention, the combination of transistors in the ARFCA is accomplished in one step, and therefore, there is no accumulation of losses and phase errors through stages as in conventional cascaded multiple stage approaches for combining large number of devices. Furthermore, each cavity is inherently less lossy than the stripline structures used in conventional approaches, and as a result, the efficiency and gain of the ARFCA can approach that of the individual transistors used.
According to another aspect of the invention, the ARFCA requires no vacuum and has no complex electrodes, circuits or windows, and therefore, various parts of the structure can be mass-produced by standard Computer Numeric Control (CNC) machines. Additionally, since combining and impedance matching is accomplished with the cavities, the ARFCA requires no other discrete passive electronic components in its RF circuitry, and as a result, the ARFCA""s reliability is increased.